Hitherto, in the shield tunnel boring machines for boring tunnels through soft and unstable ground in which the inner space of shield body is partitioned by a bulkhead at a position behind a rotary cutter head of the forward end of the shield body for boring the tunnel face ground, it has been practically impossible to directly observe or accurately reliably grasp actual state of any accidental collapses of tunnel face ground in excess of actual excavating amount being performed (which shall be referred to as "excess excavation" hereinafter) which are frequently occurring during the tunnel boring works through the soft and unstable ground. Specifically in the case of the hydraulic type boring machines utilizing generally muddy water as a liquid for hydraulic boring of the ground, such muddy water is completely opaque when excavated ground formations are mixed therewith so that direct observation of the tunnel face state can never be achieved even if an observing window is provided in the bulkhead. For this reason, there have been suggested certain measures of determining the occurrence of the excess excavation based on rapid change or increase in excavated ground formations which are drained out of the tunnel face together with the muddy water fed to the face but, with these measures, still it has been impossible to promptly determine the occurrence of the excess excavation since the measurements of the drained ground formation amount involve an inherent time lag due to existing distance between the tunnel face and actual measuring position of the drained ground formation amount and, further, practically impossible to detect or measure the location, shape and the like of the excess excavation only depending on the varying amount of the drained ground formations.